Teaching with the EarthChem Geochemical Database
Integrating Research and Education > EarthChem > Mid-Ocean Ridge Basalts > Part 2 - Analyze the data

Part 2 - Analyze the data

  1. Open the file you have just downloaded in Excel. Notice the huge number of rows! Those marine geologists have definitely been busy!
  2. Now you'll make a series of plots of the data. Start by making an x-y plot of longitude (on the x-axis) vs. latitude (on the y-axis). (Because of the large amount of data, you may want to decrease the size of the datapoint symbol used on this graph). Does the resulting graph look familiar? If not, look at a map of the world's mid-ocean ridges provided here:
    map of mid-ocean ridges
    Map courtesy of the U.S. Geological Survey

    Can you identify the Mid-Atlantic Ridge and the East Pacific Rise on your graph? These are two of the best studied mid-ocean ridges.

  3. Now make a plot of SiO2(on the x-axis) vs. MgO (on the y-axis). The easiest way to do this is to simply change the variables on your lat-long plot. Clicking on any datapoint on a plot will highlight the two columns used by surrounding those columns with colored boxes. Dragging those colored boxes to other columns will change the variables shown on the plot. Note which color surrounds the data for the x-coordinate, and which surrounds the data for the y-coordinate.

    Question 3. According to your plot, what is the range in SiO2 of MORB glasses? What could explain the higher silica glasses that are andesitic and rhyolitic in composition and trend away from the main cluster of samples?

    Question 4. Ignoring these spurious high-silica samples, what seems to be the range of MgO for the global MORB dataset? Is this variation real?--i.e., is it above or below the ~0.5 wt.% error for MgO analysis in volcanic glasses?

    Question 5. What is the likely cause for most of this geochemical variation in MgO (and SiO2)?

    Its the same process that caused the production of the anomalous high-silica glass compositions.

    Question 6. In terms of MgO content, which glass compositions will be more closely related to their original mantle sources?--low MgO, medium MgO, or high MgO?

  4. It will be of interest now to focus on the geochemical variation among the more primitive MORB liquids, but to do this, we need to find a way to correct for the effects of fractionation. Well, we're in luck! A method of fractionation correction has been developed by [Klein and Langmuir, 1987] which is based on the observation that low-pressure fractionation of a primitive basaltic liquid within an individual volcanic suite (or at a specific location) results in a more-or-less linear array ("liquid lines of descent") on major-element variation diagrams, at least over certain ranges of MgO.
    graph  of Na2O vs MgO
    Because their fractionating mineral assemblages are similar, the slopes of these liquid lines of descent for different igneous suites (or locations) are more or less sub-parallel to one another. If the slope of these linear variation arrays is known, and they are, it should be feasible to determine "calculated" values of a particular element-oxide (from different suites or locations) at a constant value of MgO. To approximate a fairly primitive basaltic liquid composition, a value of MgO = 8 wt.% is arbitrarily chosen for the value at which the "calculated" element-oxides are compared. You will perform this correction now. Its easy!

    On your spreadsheet, create three additional columns with these headings: "Fe8.0", "Na8.0", and "CaO/Al2O3", where

    Fe8.0 = FeOt + 1.664*(MgO) - 13.313
    Na8.0 = Na2O + 0.373*(MgO) - 2.98
    The formulas for Fe8.0 and Na8.0 are from [Klein and Langmuir, 1987] . CaO/Al2O3 does not change much with fractionation, so does not need to be corrected. Copy these three formulas to all the rows.
  5. Fe8.0, Na8.0, and CaO/Al2O3 are not meaningful for all values of MgO. To be able to select a range of the samples for plotting in the next step, "Select All" the cells in the spreadsheet and sort by increasing MgO.
  6. Now make three graphs showing the variation of the above three variables vs. Elevation (depth below sealevel). Plot only Fe8.0 and Na8.0 values for samples with 5.0-8.5 wt.% MgO (this is the limited range for which this calculation is valid). Similarly, plot only CaO/Al2O3 values for samples with > 5.0 wt.% MgO.

You have successfully completed the data analysis portion of this assignment. Now its time to make sense of those plots you made and learn something about MORB geochemistry! Hang in there--you're almost done!


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